ARM: EXYNOS: to declare static for mach-exynos/common.c

According to commit cc511b8d84d8 ("ARM: 7257/1: EXYNOS: introduce
arch/arm/mach-exynos/common.[ch]"), we don't need to declare extern
for exynos4_map_io(), exynos4_init_clocks(), exynos4_init_uarts(),
and exynos_init(). And the exynos4210_register_clocks() and
exynos4212_register_clocks depend on each SoC not ARCH_EXYNOS4.
So this patch fixed above.

Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>